Methods for optical inspections of surfaces in general are known from the state of the art.
E. Deutschl et al. describe in “Defect Detection on Rail Surfaces by a Vision based System”, 2004, IEEE Intelligent Vehicles Symposium, pages 507 to 511, a method for optical inspections of rail surface defects. The inspection is performed by a simultaneous illumination of the rail surface with different light colours from different directions. This method, called spectral image differencing procedure, allows the automatic detection of defects on the rail surface. The method works because the metallic rail surface reflects light of all colours substantially to the same extent. N. Bonnot et al. disclose in “Machine vision system for surface inspection on brushed industrial parts”, Proceedings of SPIE-IS&T Electronic Imaging, SPIE Vol. 5303, 2004, pages 64-72, a method for the inspection of reflecting surfaces to find surface defects like cracks. The inspection method includes an illumination procedure which uses two light sources illuminating the surface to be inspected from opposite directions at the same time.
J. Pynn et al. describe in “Automatic Identification of Cracks in Road Surfaces”, Image Processing and its Applications, Conference Publication No. 465, IEE 1999, pages 671-675, a method which is able to detect cracks in the road surface by processing digital pictures taken of the road surface.